1. Field of the Invention
The present invention relates to an intake air duct, which serves as a passage for supplying air to an engine. More particularly, it relates to an intake air duct, in which noises are reduced when taking air in.
2. Description of the Related Art
In an intake air system of an automobile engine, there arises a problem in that noises are generated at an intake air duct when taking air in. These intake air noises are irritating to ears particularly when the engines run at low revolving speeds. Hence, a side branch and/or a resonator have been disposed in the air duct conventionally, and the noises of specific frequencies, which are calculated based on the Helmholtz resonance theory, have been reduced.
However, a large branch extends as long as about 30 cm, and a resonator of a large volume enlarges as large as 14 liters. Therefore, such a noise absorption apparatus occupies a large space in the engine room. Accordingly, there may arise a drawback in that the degree of freedom decreases in boarding the other component members.
Hence, Japanese Unexamined Utility Model Publication (KOKAI) No. 64-22, 866 discloses an orifice disposed in the intake air duct. The intake air noises are reduced by throttling the intake air passage at the orifice. When the intake air passage is throttled in such a manner, the acoustic mass enlarges, and accordingly the intake air noises of low frequencies can be reduced.
Japanese Unexamined Utility Model Publication (KOKAI) No. 3-43,576 discloses an apparatus for reducing the intake air noises. The apparatus includes two intake air tubes which are connected to an air cleaner case parallelly, two branched tubes which are branched from the respective two intake air tubes respectively, a shared resonator to which the respective branched tubes are connected altogether, and an opening-and-closing valve, which is disposed on the upstream side of the connecting portion of the branched tube of one of the intake tubes, and which opens selectively depending on the running conditions.
By the apparatus disclosed in Japanese Unexamined Utility Model Publication (KOKAI) No. 3-43,576, the opening-and-closing valve is controlled so that the number of the intake air tubes are switched from one to two, or vice versa, depending on the revolving speeds of an engine. Thus, the inlet air amount is controlled depending on the revolving speeds of the engine, and accordingly the intake air noises can be reduced.
However, in the above-mentioned method in which the intake air passage is throttled, there may arise a problem in that the inlet air amount is insufficient at the high engine revolving speeds so that the engine output decreases.
In the apparatus disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 3-43,576, the opening-and-closing valve may be held between the fully closed state and the fully opened state or between the fully opened state and the fully closed state. Thus, there may be a case where the intake air tube is closed on the midway for a long period of time. In this case, although the engine is operated at low revolving speeds, the intake air noises enlarge because the acoustic mass is s mall. Accordingly, there may be a problem in that booming noises of low frequencies are heard in a passenger compartment.
Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 3-43,576 discloses to use an electronic control circuit, an electromagnetic opening-and-closing valve or a diaphragm actuator, etc., for actuating the opening-and-closing valve. When these component members are used, the number of the component members increases so that the valve complicates. Moreover, it is not preferred in view of the costs.